The subject matter described herein relates to the use of ultrasonically generated acoustic standing waves to achieve trapping, concentration, and separation of suspended-phase components and thereby remove such contaminants from a fluid medium such as water.
When particles are entrained or dispersed in a flowing fluid, aggregation of the particles to form larger clumps is typically due to some attraction or adhesion between the particles or the addition of a flocculating agent that aids in attracting and aggregating the particles. Attractive forces between the particles may be ionic or physical entanglement.
Typically, after the clumps of particles are formed in the fluid medium, a physical filtration process is utilized to separate the aggregated, agglomerated, flocculated or otherwise process-formed particle clumps. Most of the work reported in the literature for particle removal from water involves replaceable filter units consisting generally of packed cartridges, filter membranes, or special filter papers. If the separation process is a filter separation process, the physical filter media and the clumps of particles that have been separated from the fluid media are typically discarded, thus creating additional waste and increasing costs. Also, with the use of this physical filtration process, the yield of the filtrate is lessened, as some of it is used to saturate the filtering material. Further, as the filter fills up, filtration capacity is reduced, and using such filters may impose periodic stopping to remove the filter and obtain the particles trapped thereon. Finally, though particles over 10 micrometers can typically be captured by these techniques, smaller particles, such as bacterial spores in the size range of 1 micrometer, are typically not captured with sufficient efficiency.
Thus, methods are sought where continuous filtration may be carried out. Such continuous methods would be useful in various filtration applications, such as the filtering of oil from water, components from blood, tailings from water in tailing ponds, and, generally, particles from a fluid stream and immiscible or emulsified fluids from a fluid stream.
Acoustophoresis is the separation of particles and secondary fluids from a primary or host fluid using high intensity acoustic standing waves, and without the use of membranes or physical size exclusion filters. It has been known that high intensity standing waves of sound can exert forces on particles in a fluid when there is a differential in both density and/or compressibility, otherwise known as the acoustic contrast factor. The pressure profile in a standing wave contains areas of local minimum pressure amplitudes at its nodes and local maxima at its anti-nodes. Depending on the density and compressibility of the particles, they will be trapped at the nodes or anti-nodes of the standing wave. Generally, the higher the frequency of the standing wave, the smaller the particles that can be trapped due the pressure of the standing wave.
In conventional acoustophoretic devices, planar acoustic standing waves, with resonance frequencies of n*c/2 L, where n is an integer, c is the speed of sound of the fluid, and L is the length of the acoustic resonator resonator or the length of the standing wave, have been used to accomplish the separation process. However, a single planar wave tends to trap the particles or secondary fluid in a manner such that they can only be separated from the primary fluid by turning off the planar standing wave. This does not allow for continuous operation. Also, the amount of power that is needed to generate the acoustic planar standing wave tends to heat the primary fluid through waste energy.
Conventional acoustophoresis devices have thus had limited efficacy due to several factors including heat generation, use of planar standing waves, limits on fluid flow, and the inability to capture different types of materials. It would therefore be desirable to provide systems and methods of generating optimized particle clusters to improve gravity separation and collection efficiency. Improved acoustophoresis devices using improved fluid dynamics would also be desirable, so the acoustophoresis can be a continuous process.